This invention relates to an extrusion die head for making a plastic parison and in particular to a die head for making multiple parisons for producing containers having a matrix resin with a barrier forming second resin therein which is discontinuous and distributed within the matrix resin.
Certain organic solvents must be packaged and stored in containers that provide a permeability barrier which is substantially greater than is provided by conventional high density polyethylene (HDPE) containers. One way to improve the barrier is by a co-extrusion process which forms a multi-layer parison having, for example, inner and outer surface layers of HDPE and a middle barrier layer sandwiched between adhesive layers. A disadvantage with co-extrustion is the need for a separate extrusion screw for each plastic resin. Furthermore, it is extremely difficult to feed several resins to a multiple parison die head. Another way to provide a barrier container is by controlled mixing of a barrier resin, such as DuPont's SELAR RB, with a matrix resin such as HDPE. SELAR RB barrier resin is a modified nylon and an adhesive which can be dry-blended with a matrix resin, and then extrusion blow molded using a single extrusion screw. Pellets of the barrier resin are partially mixed with the matrix resin under controlled extrusion conditions to flatten and elongate to form a plurality of discontinuous, thin layer barrier walls within the matrix resin. These barrier walls overlap one another to provide permeability resistance in the wall of the finished container.
In the DuPont SELAR process the matrix resin and the barrier resin are dry mixed prior to being fed into the extrusion screw. As the pellets of the barrier resin reach their melting temperature, streaks of the barrier resin will be formed. These streaks are formed by shear forces in the resin caused by the rotating screw and are substantially circumferential. As the resin flows from the extrusion screw and through the die head to form a tubular parison, the selar streaks form barrier walls in the matrix resin which, in the finished container, overlap one another to provide permeability resistance in the container wall. A prerequisite to providing the desired permeability barrier is in controlled mixing of the barrier resin with the matrix resin. Proper mixing will produce many large, essentially two dimensional, barrier walls within the matrix resin. If the mixing is inadequate, the pellets of the barrier resin will not stretch and elongate to form the barier walls, thus resulting in little improvement in the permeability resistance of the finished article over that provided by the matrix resin itself. If the barrier resin is over mixed, the barrier walls will break apart into small particles with little improvement in permeability resistance resulting.
The proper mixing of the barrier resin with the matrix resin has been successfully accomplished with single parison die heads. However, attempts to produce barrier containers with multiple parison die heads, particularly three or more parison die heads, have had little success. In many cases, the resulting containers have no, or only little, improvement in permeability resistance. Research has indicated that one possible explanation for the difference in barrier performance of containers from single parison die heads compared to multiple parison die heads is as follows.
At the end of the screw, the flow of resin changes from primarily a circumferential flow direction to an axial flow direction as the resin flows into an adapter between the extrusion screw and the die head. During the axial flow, shear forces in the resin stream cause the generally circumferential barrier resin streaks to elongate axially, forming two-dimensional platelets in the matrix resin. In the case of a single parison center-fed die head, these platelets remain relatively undisturbed as the resin flows into the die head around the core to form a tubular parison. When the parison is blow molded, the platelets form a plurality of discontinuous, substantially two-dimensional thin layer barrier walls within the matrix resin. These barrier walls overlap one another to provide permeability resistance in the container wall.
When the resin is fed from the extrusion screw to a multiple parison die head such as a triple parison die head, the division of the resin melt into three separate conduits at the end of transfer pipe or tube disrupts the desireable structure of the barrier resin platelets. When the resin is divided, many of the platelets are broken apart such that in the finished container, there are gaps or windows in the container wall having few or no barrier walls. These containers have no or only little barrier improvement over that provided by the matrix resin individually.
It is an object of the present invention, therefore, to provide a multiple parison die head for producing containers with a barrier resin within a matrix resin in which the barrier performance in the containers is equal to or better than the barrier performance of containers produced with a single parison die head.
It is another objective of the invention that each parison from the multiple parison die head produce containers having relatively uniform barrier performance compared to one another.
It is yet another objective to achieve the above objectives in a side fed die head which offers cost advantages over a center fed die head.
The die head of the present invention meets the above objectives by producing a resin flow path which has several features similar to the resin flow path in the extrusion screw that forms the barrier resin platelets initially. This is accomplished by a downwardly spiral flow channel through which the resin melt follows a primarily circumferential path to rearrange the barrier resin platelets. During this circumferential flow, molten particles of the barrier resin will stretch, forming substantially circumferential streaks. After a predetermined length of travel in the spiral flow channel, a clearance in the die head enables a portion of the resin to flow axially downward. During the downward flow the circumferential streaks elongate to form the platelets.
The die head of the present invention includes a diverter sleeve which forms two resin flow channels in the die head that form downwardly spiral paths rotating circumferentially in opposite directions from one another. The two flow channels converge at the lower end of the diverter sleeve to form a tubular resin body within the die head that is extruded from the die head to form a tubular parison. The two spiral flow channels are concentric about a common axis with one channel radially inward of the other. As a result of the two concentric flow channels, the parsion is formed of two concentric resin layers, each having a radially extending weld line. The flow channels are configured such that the two weld lines are circumferentially displaced from one another.
By providing two concentric layers in the parison, any gaps in the overlapping barrier walls formed in one layer will likely be covered by barrier walls formed in the other channel. Furthermore, the formation of a weld line in the tubular parison that extends radially completely through the container wall can be prevented. Because no platelets are formed that extend circumferentially across the weld line, steps must be taken to prevent the formation of a radial weld line extending completely through the parison. By forming the parison with two concentric layers with the weld line of one layer displaced from the weld line of the other layer, the weld line formed in one layer will be covered by barrier walls in the other layer such that no region is created in the container wall without barrier walls. It is an advantage of this invention therefore, that no weld line is formed extending radially completely through the parison wall.
It is another advantage of the invention that with two partial weld lines rather than a single weld line, the strength of the resulting container will be increased. This strengthening effect is more pronounced with SELAR RB resin than with a homogeneous resin.
It is still another advantage of the invention that the die head has no or few resin stagnation points in the flow channels. This enables faster resin color changes to be made in comparison to standard side fed die heads.
It is still another advantage of the invention that greater uniformity in the distribution of the barrier resin is achieved. Uniform distribution of the barrier resin results in uniformity in resin stretching during blow molding and more even wall thickness distribution in the finished container.
Further objects, features and advantages of the invention will become apparent from a consideration of the following description and the appended claims when taken in connection with the accompanying drawings.